News Release Number: STScI-2004-29

NASA's Great Observatories May Unravel 400-Year Old Supernova Mystery

October 6, 2004: Four hundred years ago, sky watchers, including the famous astronomer
Johannes Kepler, were startled by the sudden appearance of a "new star"
in the western sky, rivaling the brilliance of the nearby planets. Now,
astronomers using NASA's three Great Observatories are unraveling the
mysteries of the expanding remains of Kepler's supernova, the last such
object seen to explode in our Milky Way galaxy.

Q & A: Understanding the Discovery

1.
Why is the supernova named after German astronomer Johannes Kepler?

Many people witnessed the bright "new star" that appeared in October
1604. Kepler was so fascinated that he watched the star for a year,
making detailed notes about the bright object in a logbook. While
working on the laws of planetary motion for which he is well known,
Kepler wrote a book called "De Stella Nova" ("The New Star"), in which
he describes the bright object. Scientists in the 1940s determined that
the new star was a supernova. They named it after Kepler, who was one of
the first scientists to study the object.

2.
How often does a star explode as a supernova?

In a typical galaxy like our Milky Way, a supernova pops off about every
100 years. From our earthly vantage point, we cannot see every supernova
that occurs in our galaxy because interstellar dust obscures our sight.
The Kepler supernova, which occurred 400 years ago, is the last
supernova seen inside the disk of our Milky Way. So, statistically, we
are overdue for witnessing another stellar blast. Curiously, the Kepler
supernova was seen to explode 30 years after Tycho Brahe witnessed a
stellar explosion in our galaxy. The nearest recent supernova seen was
1987A, which astronomers spied in 1987 in our galactic neighbor, the
Large Magellanic Cloud.

3.
Why are supernovas important?

All stars make heavy chemical elements like carbon and oxygen through a
process called nuclear fusion, where lighter elements are fused together
to make heavier elements. Many chemical elements heavier than iron, such
as gold and uranium, are produced in the heat and pressure of supernova
explosions. These heavy elements enrich the interstellar medium,
providing the building blocks for stars and planets, like Earth.

4.
What kind of star produces a supernova?

Two types of stars generate supernovas. The first type, called a type Ia
supernova is produced by a star's burned-out core. This stellar relic,
called a white dwarf, siphons hydrogen from a companion star, thereby
making it 1.4 times more massive than our Sun [called the Chandrasekhar
limit]. This excess bulk leads to explosive burning of carbon and other
chemical elements that make up the white dwarf.

A star that is more than eight times as massive as our Sun generates the
second type, called type II. When the star runs out of nuclear fuel, the
core collapses. Then the surrounding layers crash onto the core and
bounce back, ripping apart the outer layers.

5.
The supernova was first seen in 1604. Is that when the star exploded?

No, the explosion occurred thousands of years ago, but the light of the
explosion only reached Earth in 1604. Why did it take so long for the
light to reach us? It has to do with distance. The supernova is about
13,000 light-years away. A light-year is the distance that light can
travel in a year  about 6 trillion miles (10 trillion kilometers).
Because the supernova is 13,000 light-years away, it took 13,000 years
for light from the exploded star to reach Earth.